Method for producing magnetic core and coil assemblies with gaps in the magnetic core



H. J. PROXMIRE 3,114,196 0 ORE AND COIL Dec. 17, 1963 METHOD FOR PRDUCING MAGNETIC C ASSEMBLIES WITH GAPS IN THE MAGNETIC CORE Filed Oct.5, 1960 5 Sheets-Sheet l //7 Mentor. Har/yJP/vxm/hg y 7 I Dec. 17, 1963H. J. PROXMI'RE 3,114,195

METHOD FOR PRODUCING MAGNETIC CORE AND COIL ASSEMBLIES WITH GAPS IN THEMAGNETIC CORE Filed Oct. 5, 1960 3 Sheets-Sheet 2 //7 l/GI? 5'02".

Harry J praxw/re,

y 7 W Hate/"neg.

Dec. 17, 1963 H. J. PROXMIRE 3,114,196

METHOD FOR PRODUCING MAGNETIC CORE AND COIL ASSEMBLIES WITH GAPS IN THEMAGNETIC CORE 3 Sheets-Sheet 3 Filed Oct. 5. 1960 United States Patent.

3,114,196 METHGD FOR PRUDUClNG MAGNETIC CORE AND C(lfi. ASEBEMBLEES WITHGAPS lN THE MAGNETlC CORE Harry J. Proxrnire, Fort Wayne, Ind, assignorto General Electric Company, a corporation of New York Filed (let. 5,19%, Ser. No. cases 1 Claim. (fil. 2155.58)

This invention relates to inductive devices and to a method forproducing magnetic core and coil assemblies of inductive devices withgaps adjusted to provide a predetermined value of inductance.

In many electrical applications where the inductance of the device is :aparameter in the circuit design that cannot be neglected, it isnecessary to design the magnetic core and coil assembly with aninductance having a substantially constant predetermined value and it isdesirable that this value be maintained within specific limits. Ininductive devices, such as transformers, the magnetic core is generallycomprised of two portions which form a magnetic circuit. Conventionally,to obtain a predetermined or calculated value of inductance, one or moregaps are introduced between the two portions by providing spacers whichmaintain the portions in spaced relation.

In the past many diiliculties have been encountered in achievingaccurately spaced gaps by means of conventional spacers. Due tovariations in the permeability of the iron core and irregularities inthe laminations making up the core, it is difiicult to obtain a desiredvalue of inductance by known methods for adjusting the gap of a magneticcore. Moreover, the conventional materials used as gap spacers shrinkduring the heat treatment of the device and thus the actual value of theinductance may vary considerably from the design value because of theshrinkage.

Another method used in the past to accurately adjust the inductance of atransformer was to provide taps in the primary and secondary windingsand determine by trial and error the number of effective turns requiredin the windings that will provide the desired inductance for a givengap. This method required that the winding be provided with a pluralityof taps. Obviously, such a method is time consuming and does not resultin an economical use of materials. it is apparent, therefore, that thereis a need for a method for accurately obtaining a predetermined value ofinductance that can more economically be carried out.

Accordingly, a general object of the invention is to provide a methodfor obtaining a predetermined value of inductance in an inductive deviceby adjusting the gaps 'in the magnetic core that will achieve thedesired results set forth above.

Another object of the invention is to provide an improved method foradjusting the gap of an inductive device so that the value of theinductance can be maintained Within predetermined limits.

In accordance with the method of the invention, a core and coil assemblyis produced with gaps adjusted to provide a predetermined value ofinductance. The method involves placing at least one resilient gapspacer and coil in assembled relation with the portions of the magneticcore, forcing the core portions together to compress the resilientspacer until the desired gap is obtained and then applying a means onsaid magnetic core for maintaining said portions of said magnetic corein rigid spaced relationship to maintain a constant gap. In a morespecific aspect of the invention a thermosetting resin is applied to themagnetic core to maintain the portion in rigid spaced relationship andthe proper adjustment of the air gap is determined by measuring theinductance of the core and coil assembly.

FIG. 1 is a front elevational view partially broken away of a pair ofcoils and a core assembled in a clamp showin the resilient spacer inassembled relation with the core portions before the clamp is tightened;

FIG. 2 shows the core and coil arrangement of FIG. 1 with a coilconnected in circuit with an inductance bridge shown schematically andwith the core portions being placed under compression by the clamp, theview illustrating how the gap between the core portions is adjustedagainst the spring force exerted by the resilient spacer in accordancewith one exemplification of the invention;

FIG. 3 is a view in perspective of the core and coil arrangement of FIG.1 partially broken away and illustrating the step of applying athermosetting resin to the core portions in the vicinity of the gapafter the desired gap has been obtained by the step illustrated in FIG.2;

PEG. 4 illustrates the curing step of the foregoing exemplification ofthe invention and shows the transformer after the thermosetting resinhas been applied in accordance with the step illustrated in FIG. 3,being heated in an oven to cure the thermosetting resin;

FIG. 5 is a front elevational view of an electromagnetic inductiveapparatus having a shell-type magnetic core formed of E-shapedlarninations and having a resilient spacer in assembled relation withthe core portions to illustrate another exemplification of theinvention;

FIG. 6 is a front elevational view partially broken away of the core andcoil assembly shown in FIG. 5 in which the apparatus is shown connectedto an inductance bridge to illustrate the step of adjusting the gap toobtain a predetermined value of inductance; and

FIG. 7 is a side elevational view partially broken away illustrating howthe resin is applied to the core portions in the vicinity of the gapsaccording to the method of the invention.

Referring now to FIGS. 1, 2, 3 and 4 and more particularly to theperspective view of FIG. 3, the method of the present invention isillustrated in connection with a core and coil assembly 10 comprising asintered iron magnetic core 11 and two coils 12, 13. It will be seenthat the magnetic core is formed of two portions which are spaced fromeach other to provide a gap in the magnetic circuit and are made up ofsix C-type isintered iron core elements 14, -15, 16, 17, 18, 19 and 20,21, 2-2, 23, 24, 25, respectively. The upper and lower portions of themagnetic core 11 are spaced from each other by resiilent gap spacers. Inthe exemplification of the invention shown in FIGS. 1-4, the resilientspacers 26, 27, are comprised of elongated strips 28, 29 of resilientmaterial, such as rubber, neoprene and the like, which are sandwichedbetween two'shims 30', 31 and 32, 33, respectively. The shims 30, 31,32, 33 are used for the purpose of maintaining the alignment of theC-shaped core elements 14, 15, 16, 17, 18, =19 and 2t}, 21, 22, 23, 24,2:5, and for the purpose of aiding in dete'rmining the initial spacingrequired. Two insulating members 34, 35 provide support for the coilassemblies and also insulate the sides of the coils 12, 13 from themagnetic core 11.

A clamp 36 is used in the illustrative embodiment of the invention inthe step of forcing or drawing of the portions of the magnetic core 11against the resilient force of the spacers 26, 27 to accurately adjustthe length of the gap. The clamp 36 simply comprises two parallel plates37, 38, two clamping screws 3, 4i) and wing nuts 41, 42, the clampingscrews 39, to being secured at one end to the lower plate 38. Suitablewashers 4-3, 44 are provided to facilitate tightening of the wing nuts41, 42. As shown in FIG. 1, the wing nuts 41, 42 are drawn up on theclamping screws 39, 44) so that no compressive force is exerted againstthe portions of the magnetic core 11.

Referring now to FIG. 2, as the wing nuts 41, 42, are drawn downward, itwill be seen that upper and lower portions of the magnetic core 111 areplaced under com: pression. Since this compressive force applied by theclamp is resisted by the spring force of the resilient spacers, it willbe seen that very fine adjustments in the gap spacing are possible. Itwill be appreciated that the compressive force required to draw theportions of the magnetic core 11 together and compress the resilientspacers 26, 27 may be provided by other suitable means.

To determine the precise gap spacing that will provide the predeterminedvalue of the inductance, the gaps are progressively adjusted as theforce exerted by the clamping means is increased. The value of theinductance is measured while these adjustments are made. The coil 13 isconnected in an alternating current inductance bridge circuit 46 shownschematically in FIG. 2. The bridge circuit 46 includes a decade type ofinductance switch 47 that provides predetermined value of the inductanceagainst which the inductance of the core and coil assembly is compared,a galvanometer 4S and a switch 49. Thus, while the gap is beingadjusted, the actual inductance readings for a given adjustment arecompared against the predetermined standard value provided by the decadeinductance switch 47. When the two values of inductance aresubstantially equal, as shown by the galvanometer readings, the desiredgap has been obtained.

After the desired gap is obtained by the preceding step, the next stepof the invention involves applying to the magnetic core 11 a means formaintaining the core portions of magnetic core 11 in rigid spacerelationship.

Preferably, as shown in FIG. 3 thermosetting resin 51 is applied to thecore 11 in the vicinity of the gaps so that upon curing the resin willform a rigid connection between the portions of the core 11 and maintainthe portions in rigid spaced relation. The thermosetting resin 51 may beapplied by means of a caulking gun 52, only a portion of which is shown.In this illustrative embodiment of the invention, a heat curablepolyester resin, propylene glycol ma'leate adipate was used. The resinwas cured for a period of two hours at a temperature of 125 degreescentigrade. Resins containing polymerizable acrylic acid type of estershave good bonding properties. However, it will be appreciated that manyother types of resins, such as the epoxy resins, may be used. Further,it will be appreciated that the portion of the core 11 may also bemaintained in rigid spaced relationship by clamping frames and othermechanical means.

In FIG. 4 the core and coil assembly .10 and clamp 36 are shown in anoven 52 heated by the electrical element 53. Whether or not a heatingstep is required in a practice of the invention will depend upon whetherthe thermosetting resin employed requires heating to effect a cure. Ifthe resin used is curable at room temperature, the curing step,obviously, becomes unnecessary. In the hereinafter describedexemplification of the invention, a polyester resin having anaerobiccuring characteristics was used, and consequently it was not necessaryto heat the core and coil assembly.

Referring now to FIGS. 5, 6 and 7, the method of the present inventionis illustrated in connection with a reactor 55. The reactor 55 iscomprised of a coil 56 wound on a spool 57 and a magnetic core 58 formedof two oppositely disposed stacks of E-shaped laminations 59, 60. Thelaminations are firmly held together by a clamping member 61substantially channel-shaped and formed with two tabs 62, 63 at eachend. Resilient spacers 64, 65 are interposed between the ends of theouter legs of lamination stacks 59, 60.

As shown in FIG. 5, the reactor 55 is loosely held in a clamp 36 whichis identical to the clamp 36 of FIGS. 1-4, like reference numerals beingused to identify cor responding parts. The clamp 36 consists of the twoparallel plates 37, 38, the two clamping screws 39, 4t secured to thebottom plate 38, and the wing nuts 41, 42. In the view shown in FIG. 6,the reactor 55, the tabs 62, 63 of the clamping member 61 are foldedover, and the two stacks of laminations 59, 60 are under compression.Further, there is shown in FIG. 6 a schematic circuit diagram of aninductance bridge which is substantially the same as the circuit shownin FIG. 2. Like reference numerals are used to identify thecorresponding components of the circuits.- The proper value of theinductance is determined by adjusting the gaps until the galvanometerindicates that the values of inductance of the predeter mined standardvalue set on the decade inductance switch 47 and the inductance of thereactor 55 are substantially equal.

The terminals of the bridge circuit 46 were connected across analternating current supply of four volts at 3500 cycles per second. Whenthe predetermined value of the inductance was obtained, a small amountof resin was introduced into the aperture 67 formed in the clampingmember. The resin 66 is attracted by capillary action between theclamping member 61 and the lamination stacks 59, 69 so that when theresin is cured to an infusible and insoluble mass, the two laminationstacks will be held in rigid spaced relationship.

In the illustrative embodiment of the invention shown in FiGS. 5-7, ares-in 6 comprising a polymerizable acrylic acid ester having anaerobiccharacteristics was used. Such polymerizable compositions which can beused are fully described in U.S. Patent No. 2,628,178Burnett et al.These resinous compositions readily penetrate between adjoining surfacesby capillary action and then rapidly polymerize t-o form a solidinfusible bond between the surfaces because of the absence of air. Byusing an anaerobic synthetic resin it is possible to eliminate theheating step since the curing of such resins can be readily etlectedwithout the application of heat.

The method of the present invention provides the distinct advantage overother known methods for producing magnetic core and coil assemblies withgaps adjusted to provide a predetermined value of inductance of therated value in that more precise adjustments are rendered economicallyfeasible even in such difiicult magnetic core structures such as thesintered iron core type of transformer illustrated in FIGS. 1-4. Themethod of this invention makes it possible to maintain the ratedinductance within very close limits. As an example, with the method ofthe present invention it was possible to maintain the primary inductanceof the core type of transformer illustrated in FIGS. 1-4 within twopercent of the predetermined value for factory assembled transformers.When methods of the prior art were employed for adjusting the gap,inductance could only be main tained within twenty percent of the ratedvalue. To obtain a value of inductance within two percent of the designvalue by the trial and error method of employing multiple taps in awinding, it was found that approximately ten taps were required aboveand below the nominal voltage rating or a total number of twenty taps.Thus, the multiplicity of taps required rendered this method impracticaland uneconomical from a manufacturing standpoint. Moreover, it will beappreciated that the gap obtained in accordance with the invention isnot dependent upon the spacer material or the mechanical structure ofthe unit when a resin has been used to bond the core. Thus, the methodof the present invention insures the gap will be permanently maintainedat its preset spacing during the service life of the device. Further, itwill be apparent that the present method eliminates the need forproviding multiple taps in the windings of an electromagnetic inductivedevice to which in the past were used to fix the value of theinductance.

While I have illustrated embodiments of my invention in connection totwo different types of core structures, it will be understood that myinvention is not limited to use with such core constructions. My methodof producing magnetic core and coil assemblies may be used with anymagnetic core in which a gap is formed between portions of the corewhich can be drawn together to compress a resilient spacer interposedbetween the portions.

Although I have shown and described herein particular embodiments of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications can be made therein without departing from theinven tion. Therefore, it is aimed in the appended claim to cover allsuch changes and modifications as fall Within the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A method of producing a magnetic core and coil assembly with a gapadjusted to provide a predetermined value of inductance for theassembly, said magnetic core comprising a plurality of C-shaped coreelements oppositely disposed with respect to each other, said methodcomprising: assembling the C-shaped core elements with a resilient gapspacer sandwiched between two essentially rigid spacers of nonmagneticmaterial in the gap between the G-shaped core elements, placing theC-shaped core elements in a clamp to adjustably draw the core elementstoward each other against the gap spacers, connecting the coil assemblyin circuit with an inductance bridge for determining the inductance ofsaid core and coil assembly,

adjusting the clamp to draw said core elements progressively toward andaway from each other against the spring force of the resilient gapspacer until the predetermined value of inductance is obtained, applyinga thermosetting resin to the core portions in the vicinity of the gap,curing said thermosetting resin to an infusible state to rigidly bondsaid core elements thereby to maintain the gap therebetweensubstantially constant, and removing said core and coil assembly fromsaid clamp.

References Cited in the file of this patent UNITED STATES PATENTS2,055,175 Franz Sept. 22, 1936 2,165,055 Kafka July 4, 1939 2,180,759Kneisley Nov. 21, 1939 2,318,095 Putman May 4, 1943 2,319,775Mitter-maier May 18, 1943 2,367,591 McAllister Jan. 16, 1945 2,445,408Root July 20, 4948 2,550,127 Specht Apr. 24, 1951 2,558,110 Stein June 26, 1951 2,586,320 Ford Feb. 19, 1952 3,043,919 Tannenbaum et a l. July10, 196-2

